Unsaturated polyester resins are well-known and useful materials for making molding compositions with broad application in the manufacture of automotive and other products. Unsaturated polyester resin compositions typically used for sheet molding compounds (SMC's) are prepared from the polycondensation of propylene glycol and maleic anhydride. Other polyesters are similar in that some of the maleic anhydride is replaced with another anhydride or dicarboxylic acid, or some of the propylene glycol is replaced with another glycol to achieve different properties. Common to these materials is the presence of reactive unsaturation in the polyester resin, a result of the maleic anhydride component or reactively unsaturated dicarboxylic acids used in the polycondensation. In the final polyester, much of the used in the polycondensation. In the final polyester, much of the unsaturation is internal in the resin and not exclusively at the ends.
One of the main deficiencies of cured unsaturated polyesters and fiber reinforced polyesters (FRP's) made from them is the inherent brittleness of the polymer matrix. Cracking, at room and elevated temperatures, is a principal cause of failure in production and in service of the molded parts. Improvements in crack resistance in resin materials for SMC's have been reported by the incorporation of flexible ethers (diethylene glycol) or acids (adipic acid) in the unsaturated polyester. This approach using the so-called flexible polyesters has been only moderately successful, since the modulus and high temperature properties of the molded composite significantly decline with increasing amounts of these materials, and frequently they are faulted as sacrificing too much in their mechanical properties and dimensional stability.
Further, rigid unsaturated polyesters which contain a high fumarate content have traditionally given the best surface quality when used with common low profile additives (LPA) to control shrinkage. Consequently, they have been the materials of choice for Class A automotive panels. As well as showing a great loss of mechanical properties, flexible polyesters sacrifice surface quality even when used with common LPA's. This has discouraged the use of such flexible unsaturated polyesters for vehicle panels.
A number of strategies for improving the toughness and crack resistance of polymeric materials have been demonstrated in thermoset epoxies. The most common and successful technique is the creation of a second elastomeric phase through a phase separation process during cure. This technique has found practice but limited success in unsaturated polyesters. In addition to limited improvements in mechanical properties, the addition of reactive liquid rubbers typical of this approach leads to high viscosities which are unacceptable for the manufacture of SMC's by traditional means. Further, the utility of rubber and liquid rubber additives is limited by the incompatibility of these materials with polyester resins, particularly as molecular weight of the additive increases. If a resin mixture or even an SMC paste mixture containing the rubber additive is allowed to stand under ordinary storage conditions, the rubber will ultimately separate. Finally, compositions incorporating such materials exhibit moderate shrinkage which is not easily controlled by the LPA, resulting in a surface quality unacceptable for automotive surface parts.